The WADA (West Africa Data Base on Antiretroviral Therapy) Collaboration is a unique collaboration among cohorts in West Africa with a mission to conduct hypothesis-driven epidemiological research on the prognosis and outcome of HIV type 1 and 2-infected people, including adults, pregnant mothers, and children. The research will focus on scientific questions requiring a large sample size of patients which the contributing cohorts cannot answer individually and which do not overlap with existing projects. Activities during Year 1 will focus on cross-sectional analyses of existing data, a comprehensive examination of the available data in terms of completeness of core items at baseline and during follow-up, and the development of instruments to standardize, harmonize and improve data collection across sites. The research agenda and specific hypotheses for Years 2-5 will be finalized based on the results of Year 1 activities, with a focus on the following four areas: programmatic issues (access to ant- retroviral treatment and losses to follow up);monitoring and outcomes of treatment in adults, children and pregnant women;clinical response, with a special interest for tuberculosis, opportunistic infections, immune reconstitution syndrome and hepatitis B coinfection;care of HIV-2 infection. Additional research questions will be focused in the subsequent years, with the expectation to address around 30 completed projects and analyses over five years. A total of 17 cohorts in six major cities in six West African countries - each ongoing and with their own internal governance structure and financial support - have agreed to contribute data collected already over the past years from a collective number of 2,345 HIV-infected children and 33,245 adult patients. The regional coordinating centre for this application will be located in Bordeaux, France. This university team (Prof Francois Dabis, ISPED, Universite Victor Segalen) has a long lasting experience in epidemiological HIV cohort research in West Africa, with an office in Abidjan, Cote d'lvoire. It also has extensive experience with international collaborations of this kind including the Ghent Group on HIV in Women and Children and the ART-LINC Collaboration. ISPED will be responsible for the implementation of the WADA Collaboration. Data will be collected from affiliated cohorts every year and once merged into a main WADA database, data extracts will be made for the various projects and analyses. This process will be repeated annually allowing for the analysis of the most contemporary data collected in the affiliated cohorts. WADA is an unprecedented undertaking in West Africa, with a unique design, lead by investigators with extensive experience, that will advance the public health knowledge required to better understand and treat HIV-infected persons of all ages, genders and backgrounds in sub-Saharan Africa, the continent with the largest unmet needs but with onqoing, rapid and massive roll-out of HIV treatment programmes.